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To study how different, constant photoperiods affect endogenous breeding cycles in the threespine stickleback, adult males were kept in separate aquaria for up to two years under light:dark (LD) 12:12 hr or LD 16:8 or for eleven months under continuous light. Nests built were destroyed weekly and fish were considered breeding as long as they kept rebuilding. Under continuous light most males nested for a much longer first period than under the other photoperiods. Most fish under both LD 12:12 and LD 16:8 exhibited breeding cycles shorter than one year. After the first or second breeding cycle, many fish, especially under LD 12:12, exhibited irregular breeding patterns. There were no significant differences in cycle length or proportion of time spent in breeding condition between these two photoperiods. The lack of these differences and the surprising observation that some fish under LD 12:12 remained in breeding condition for a very long period, suggest that a circannual cycle in photosensitivity is not the whole explanation for the endogenous control of breeding in sticklebacks.